The "long fiber-reinforced polyamide resin composition" is obtained by, for example, impregnating a reinforcing long fiber bundle with a molten polyamide resin. Reinforced molded articles obtained from the long fiber-reinforced composition exhibit prominently improved impact resistance as compared with conventional short fiber-reinforced molded articles. The reason is presumably that the long fiber-reinforced composition (pellet) contains reinforcing fibers having a length substantially equal to that of the pellet. Because of its excellent properties, the long fiber-reinforced polyamide resin composition has been widely used.
However, the polyamide resin is desired to be further improved in lightweight properties, hygroscopicity and cost performance (because of its high cost), though it has high heat resistance. In addition thereto, the polyamide resin is unexpectedly brittle when repeatedly subjected to shock (that is, poor in repeated impact properties), though it is good in ordinary impact resistance. Therefore, the use of the polyamide resin composition is specifically limited.
In this connection, Japanese Patent Laid-Open Publication No. 58458/1985 proposes a composition comprising a specific modified polypropylene a specific polyamide and a fiber reinforcement. It is described in this publication that a composition which has heat resistance almost equal to that of polyamide and is highly improved in lightweight properties, hygroscopicity and cost performance can be obtained according to this invention. However, a means to prepare the composition disclosed in the publication is only a method of melt kneading a resin and preliminarily chopped glass fibers by an extruder or the like. According to the additional tests, molded articles obtained from the composition is entirely insufficient not only in repeated impact resistance properties but also in impact resistance.
The present inventors have already invented a novel long fiber-reinforced resin composition, that comprises a resin matrix of a "polymer alloy" made from a polyamide resin and a modified olefin crystalline polymer prepared by graft modification with unsaturated carboxylic acids, particularly, a modified propylene crystalline polymer, to which long fiber reinforcement is homogeneously added. According to this invention, there can be obtained a long fiber-reinforced resin composition which is excellent in lightweight properties, cost performance and ordinary impact resistance and, moreover is remarkably improved in repeated impact resistance.
In fields where much higher performance is required for molded articles produced from long fiber-reinforced compositions, however, there is room for improvement in the dispersibility of the long fibers in the molded articles to thereby enhance mechanical properties such as tensile strength. Further, the glass fiber reinforcement in the form of small bundles remains in the molded articles because of lack of loosening of bundles, and, thefore, the molded articles sometimes have bad appearance. Furthermore, there remains room for further improvement in repeated impact resistance of the molded articles.
As another prior art technique, a resin composition comprising a thermoplastic resin containing fibers (first resin) and other resin (second resin) having a melting point lower than that of the first resin has been proposed in Japanese Patent Publication No. 60780/1993. In this publication, it is described that the second resin is melted prior to melting of the first resin and includes the first resin. Therefore, folding of the fibers is inhibited by the first resin, and as a result, the molded article can be improved in mechanical strength, rigidity and heat resistance. According to the studies by the present inventors, however, the degree of the improvement in the mechanical strength of the molded article is still insufficient, though the folding of the fibers in the molded article is undoubtedly improved (reduced). Further, the repeated impact resistance is never improved, though the ordinary impact resistance is improved.
As is apparent from the above, a fiber-reinforced resin composition showing high dispersibility of fibers in the molded article and excellent in all of the lightweight properties of the molded article, cost performance, mechanical strength and the repeated impact resistance properties has been eagerly desired, but no satisfactory fiber-reinforced resin composition has been obtained yet. Accordingly, it is an object of the present invention to provide a fiber-reinforced composition excellent in these properties.